Field of the Invention
This invention relates to a novel protein/antigen, aquaporin 2, from Rhipicephalus microplus and immunogenic compositions containing this protein/antigen. This invention also relates to methods for protecting cattle from bovine babesiosis, a disease caused by Babesia bovis, which is transmitted to cattle during feeding of R. microplus, the vector of B. bovis. 
Description of Related Art
Aquaporin (AQP) is a transmembrane protein family that forms pores to transport water and small solutes across cellular membranes (Borgnia, et al., Annu. Rev. Biochem. 68:425-458 (1999)). Members of the AQP family have been identified throughout the plant and animal kingdoms (Gonen and Walz, Quarterly Reviews of Biophysics 39:361-396 (2006)). AQP structures are conserved among species, having six transmembrane domains that are connected by two intracellular loops and three extracellular loops. Two aspargine-proline-alanine (NPA) motifs are considered AQP signature motifs and are located at the protein portion that interacts to form a pore (Borgnia, et al. (1999)). A total of 13 AQP members have been identified so far and classed into two subsets: those permeated by water and those permeated by water plus other small molecules, such as glycerol and urea (Borgnia, et al. (1999); Gonen and Walz (2006)).
Numerous members of the AQP family have been identified in arthropods in the last few years because of the availability of several arthropod genomes and genetic information, such as transcriptomes and cDNA libraries (Benoit, et al., J. Comp. Physiol. B. 184(7):811-25 (2014)). Studies have shown that AQPs play a pivotal role in arthropods, especially in blood-feeding species, such as mosquitoes and ticks. In fact, blood-feeding arthropods have become a model for AQP research because of the critical relevance that osmoregulation plays during feeding (Campbell, et al., J. Comp. Physiol. B. 178:935-955 (2008)). Tick females ingest up to 100 times their body weight in blood, returning approximately 75% of the ingested water and ions via their saliva into the host (Kaufman and Philips, J. Exp. Biol. 58:523-536 (1973)). Therefore, the osmoregulatory system of ticks is central for their life cycle and has implications for efficient feeding and subsequent generation of viable offspring.
The cattle tick Rhipicephalus (Boophilus) microplus is one of the most economically important ectoparasites of bovines, as it is the main vector implicated in the transmission of the apicomplexan protozoan Babesia bovis, the etiological agent of bovine babesiosis (Friedhoff, K T, Transmission of babesia, pp. 23-52 in Babesiosis of Domestic Animals and Man, ed.: Ristic, CRC Press (Boca Raton, Fla.) (1988)). R. microplus adult females acquire B. bovis merozoites by ingesting blood from an infected bovine and pass the protozoan transovarially to their larval progeny which then transmit B. bovis sporozoites to naïve cattle during subsequent feeding (Friedhoff (1988); Mahoney and Mine, Res. Vet. Sci. 26:253-254 (1979); Bock, et al., Parasitology 129(Suppl):5247-5269 (2004)). Currently, control of bovine babesiosis relies mainly on targeting tick populations and on the use of live attenuated vaccines in most endemic areas (Friedhoff (1988); Mahoney, et al. (1979); Bock, et al. (2004)). The control of R. microplus is mainly based on the use of acaricides and, to a much lesser extent, by anti-tick vaccination (Fragoso, et al., Vaccine 16:1990-1992 (1998); Jonsson, et al., Vet. Parasitology 88:275-285 (2000)). However, the efficacy of commercial anti-tick vaccines is inconsistent in different regions of the world. Furthermore, the recent development of tick populations resistant to acaricides represents a serious threat to the cattle industry (Miller, et al., J. Med. Entomol. 42:912-917 (2005); de la Fuente, et al., Animal Health Research Reviews 8:23-28 (2007)). Additionally, the reemergence of R. microplus in areas that had been considered to be free of this tick, such as the regions outside the permanent quarantine zone in south Texas, is causing concerns about the reintroduction of B. bovis into areas currently free of bovine babesiosis. Exposure of naïve cattle in these areas to B. bovis would lead to significant mortality since no herd immunity is present in the population.
U.S. Pat. No. 8,722,063 covers R. microplus aquaporin 1 (RmAQP1) and the use of RmAQP1 as an antigen to reduce tick viability and reproduction.
A second aquaporin protein from R. microplus (RmAQP2) was recently identified. See, Guerrero, et al., Insect Biochemistry and Molecular Biology 35:585-595 (2005); and Guerrero, et al., Parasites & Vectors 7:475 (2014). RmAQP2 is 42% identical to RmAQP1 at the amino acid level but 85% identical at the amino acid level to a Dermacentor variabilis aquaporin. See, Guerrero, et al. (2014). RmAQP1 is expressed in tick synganglia. In contrast, RmAQP2 is expressed in salivary glands.
As such a vaccine that is effective against R. microplus that would also prevent transmission of tick-borne pathogen, B. bovis, is vital. Because intake of blood meals leading to full engorgement of adult females and generation of viable larval offspring are critical steps of the tick life cycle, perhaps attempts to disrupt the feeding process can result in a vaccine with better efficacy than the currently marketed vaccines.